Why is the largest land animal currently the elephant? Why are they so lethargic, going miles out of their way so that they don't have to walk uphill at even the slightest grades?

How is it that there were so many land animals even larger than them at one time? If weight (volume) increases with the cube of size, but strength of muscles (cross section) only increases with the square of size... wouldn't the largest carnivores be little mewling kittens compared to a raging elephant?

There is an interesting and absolutely disdained hypothesis that would explain how sauropod biology could ever work (how the fuck could their heart pump blood to their brain, etc),and how 15m wide pterodactyls could fly: atmosphere was way thicker back then (let's say twice as dense as nowadays).